Numerous unsuccessful attempts have been made in the past to efficiently reprocess a feedstock of waste plastics back into a liquid hydrocarbon product that has valuable and useful properties. A considerable tonnage of waste commodity plastics, or polymers, are improperly disposed of each year. In previous years, there has been considerable improvement in the collection and recycling of common waste polymers. However, the recycling and reuse of waste polymers, as now practiced without subsidies have proven to be uneconomic, forcing the abandonment of many recycling efforts. The recycling of waste polymers as fuel has also not proven practical because of the inability to collect sufficient quantities to sustain operation of the recycling machinery.
Thermolysis refers to the dissociation or decomposition of chemical compounds by heat and, in particulars is a process that has been described whereby plastic waste material is converted to liquid fuel by thermal degradation (cracking) in the absence of oxygen. Plastic waste is typically first made molten within a stainless steel chamber under an inert purging gas, such as nitrogen. This chamber then heats the molten material to a gaseous state that is drawn into a catalytic converter and cracked to form carbon chain lengths of a certain desirable range.
Hot pyrolytic gases of the desired carbon length range are then condensed in one or more condensers to yield a hydrocarbon distillate comprising straight and branched chain aliphatics, cyclic aliphatics and aromatic hydrocarbons. The resulting mixture is roughly equivalent to regular diesel fuel after the placement of additives and the implementation of other minor downstream processing actions.
Diesel fuel is a blend of hydrocarbon compounds known as distillates that are heavier than gasoline but lighter than lubricating oil. Diesel is a mixture of straight-chain and branched alkanes, cyclic saturated hydrocarbons and aromatics. Diesel fuel is designed to operate in a diesel engine only, where it is injected into the diesel engine combustion chamber wit compressed, high-temperature air and ignites spontaneously. Conversely, gasoline in a petrol engine is ignited by spark plugs. Diesel fuel produced by pyrolysis and other methods must meet a range of composition requirements before being certified for sale in a number of countries.
Existing thermolysis system based plants are in operation but display a series of significant shortfalls and disadvantages, which ultimately result in inefficient production and throughput of liquid hydrocarbons from waste plastic materials. A primary disadvantage is the process of loading plastic waste manually, and also the system being unable to receive variable sizes of plastic waste feedstocks. Existing Systems that receive manually loaded blocks or rolls of plastics leave a great deal of excess air space in the pyrolysis chamber, which must be sealed to allow purging of atmospheric gas and melting of the material. Once sealed, no further waste can be added to the chamber which otherwise has space for more material once it is melted. The inability to fill the pyrolysis chamber to its maximum reduces the speed of the entire process and ultimately the throughput of these known thermolysis systems.
Typically, the known thermolysis plants have two pyrolysis chambers, known as twin-chamber systems, which work together at an approximately equal rate. Of particular issue with the set up of existing pyrolysis chambers is that fuel may only be produced in a batch type basis. Once both chambers complete the pyrolysis of waste material in unison, it is necessary to wait for each to cool before unwanted carbonaceous char can be removed from the internal base of each chamber. It is therefore presently necessary to wait for each chamber to cool and be hand cleaned of char before the chambers can be used again for second and subsequent batches. Whilst it may be necessary in a batch process according to one aspect of the present invention to manually vacuum out char by access through the manhole (inspection port) in the chamber lid, in a preferred continuous process according to another aspect of the invention this issue is addressed below. In addition to this set-up limitation associated with batch processes, the physical size of the existing chambers greatly limits the batched throughput capacity of waste materials.
Of further issue with existing thermolysis systems are higher frequency clean out times of chambers, poor heat transfer within the waste feedstock itself increased process time per chamber load and also the expense and fouling of consumable catalysts. The final issue is the quality of the final fuel product including a measure of diesel fuel's ability to reduce wear on contacting solid surfaces found in some fuel pumps and injectors.
Of further issue with existing thermolysis systems is that fuel produced is often of a variable standard and cannot be used in engines and other machinery that lacks a tolerance to such fuel inconsistency. Of particular concern is the property of the fuels produced by these existing processes, which have a flash point generally varying between 25° C. and 45° C., which are too low due to the high presence of light fuel fractions. Such fuels are therefore unsuitable for the expected use in all regions of the world, and applications other than stationary engines. Other prior art processes produce heavier oils which are also unsuitable for diesel engine operation.
The above discussion and any other discussion of pre-existing documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material forms a part of the prior art base or the common general knowledge in the relevant art in Australia or elsewhere on or before the priority date of the disclosure and claims herein.
In light of the foregoing discussion, an object of the present invention is to provide a thermolysis process that has an increased throughput of liquid hydrocarbons that are of consistent quality, using waste plastic stockfeed.
A further object of the present invention is to at least alleviate or overcome at least one disadvantage associated with the prior art.